Rare earth/iron fluoride and methods for making and using same

ABSTRACT

A particulate mixture of Fe 2  O 3  and RE 2  O 3 , where RE is a rare earth element, is reacted with an excess of HF acid to form an insoluble fluoride compound (salt) comprising REF 3  and FeF 3  present in solid solution in the REF 3  crystal lattice. The REF 3  /FeF 3  compound is dried to render it usable as a reactant in the thermite reduction process as well as other processes which require an REF 3  /FeF 3  mixture. The dried REF 3  /FeF 3  compound comprises about 5 weight % to about 40 weight % of FeF 3  and the balance REF 3  to this end.

CONTRACTUAL ORIGIN OF THE INVENTION

The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-82 between the U.S. Department of Energy and Iowa State University, Ames, Iowa, which contract grants to the Iowa State University Research Foundation, Inc. the right to apply for this patent.

FIELD OF THE INVENTION

The present invention relates to a rare earth/iron fluoride compound useful as a reactant in the thermite reduction process and other metallothermic processes and to a method of making the rare earth/iron fluoride compound.

BACKGROUND OF THE INVENTION

The thermite reduction process described in the Schmidt et al. U.S. Pat. No. 4,612,047 is useful in the production of rare earth/iron alloys from which high performance permanent magnet alloys (e.g. Nd₂ Fe₁₄ B) are in turn made. The thermite reduction process involves preparing a mixture of a rare earth fluoride, iron fluoride and calcium metal, heating this mixture under reducing conditions to a temperature sufficient to react the fluorides with the calcium metal to form a rare earth/iron alloy and a calcium fluoride slag, and then separating the alloy from the slag.

The rare earth fluoride (e.g., NdF₃) used as one reactant in the thermite reduction process is typically prepared by wet or dry hydrofluorination of the corresponding trivalent rare earth oxide (e.g., Nd₂ O₃) The iron fluoride (e.g. FeF₃) employed as another reactant (thermal booster) in the thermite reduction process is typically prepared by dry hydrofluorination of relatively expensive ferric chloride (FeCl₃) or by wet hydrofluorination from a concentrated ferric chloride solution. Attempts have been made to reduce the cost of the iron fluoride reactant by dry hydrofluorination of ferric oxide (Fe₂ O₃) which is much less expensive than ferric chloride. Although various attempts have been made to dry hydrofluorinate Fe₂ O₃ at temperatures up to about 600° C., none has been successful in producing iron fluoride acceptable for use in the thermite reduction process.

There nevertheless remains a continuing desire to reduce the cost of the iron fluoride reactant as a means of reducing the cost of the thermite reduction process. Moreover, any reduction in the number of individual reactants required to carry out the thermite reduction process and/or the number of processing steps required to make the rare earth fluoride and iron fluoride reactants for that process would be desirable for the same reason.

An object of the present invention is to provide a new, lower cost, wet process of making a rare earth/iron fluoride compound useful as a reactant in the thermite reduction process.

Another object of the invention is to provide a novel "solid solution" rare earth/iron fluoride compound that is useful as a reactant in the thermite reduction process in lieu of the separate rare earth fluoride reactant and iron fluoride reactant heretofore employed.

SUMMARY OF THE INVENTION

The invention envisions a new wet (aqueous) method for preparing a novel rare earth/iron fluoride compound useful as a reactant in the thermite reduction process as well as other metallothermic processes. In accordance with the method of the invention, a particulate mixture of Fe₂ O₃ and RE₂ O₃ (where RE is a rare earth element) in selected proportions is prepared and reacted with HF acid present in an amount exceeding the stoichiometric amount needed to fluorinate the Fe₂ O₃ and RE₂ O₃. The HF acid is present in an excess amount effective to yield a novel rare earth/iron fluoride compound comprised of FeF₃ present in solid solution in the crystal lattice of REF₃ (as indicated, for example, by an X-ray diffraction pattern of the compound in dried form). The novel rare earth/iron fluoride compound is formed as an insoluble, hydrated fluoride salt which is then subjected to a suitable drying treatment.

The dried rare earth/iron fluoride (REF₃ /FeF₃) compound is useful as a low cost reactant in the thermite reduction process. To this end, the dried compound preferably comprises about 5 weight % to about 40 weight % FeF₃ in solid solution in the REF₃.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow diagram illustrating method steps for making the rare earth/iron fluoride compound in accordance with the invention.

FIGS. 2, 3 and 4 are X-ray diffraction patterns of a NdF₃ /FeF₃ compound, DyF₃ /FeF₃ compound and GdF₃ /FeF₃ compound in dried form prepared in accordance with Examples 1, 2 and 3, respectively.

FIGS. 5, 6, 7 and 8 are X-ray diffraction patterns of dried NdF₃, FeF₃, DyF₃, and GdF₃, respectively, for comparison to the respective diffraction patterns of FIGS. 2, 3 and 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the various steps involved in practicing one embodiment of the method of the invention are illustrated. In particular, particulates of Fe₂ O₃ and RE₂ O₃, where RE is a rare earth element such as Nd, Dy, Gd, Ho, Er, Lu, La, Sc, and Y, are initially mechanically mixed in selected proportions to provide a starting particulate mixture. The Fe₂ O₃ particulates are employed in a size range of about -10 to about -30 mesh and are commercially available from several chemical suppliers, such as Pfizer Minerals, Pigments & Metals Division, 235 E. 42nd St., New York, N.Y. 10017 and J. T. Baker, Inc., 222 Red School Lane, Phillipsburg, N.J. 08865. The RE₂ O₃ particulates are used in a size range of about -10 to about -30 mesh and are commercially available from various rare earth oxide suppliers such as Molycorp Inc., 709 Westchester Avenue, White Plains, N.Y. 10604, Research Chemicals Co., P.O. Box 14588, Department R, Phoenix, Ariz. 85063 and Sausville Chemical Co., 176 Saddle River Avenue, Garfield, N.J. 07026.

In general, the RE₂ O₃ particulates useful in practicing the invention are selected from the trivalent lanthanide rare earth oxides (e.g., La₂ O₃, Nd₂ O₃, Dy₂ O₃, Gd₂ O₃, Ho₂ O₃, Er₂ O₃, Lu₂ O₃, Sc₂ O₃ and Y₂ O₃, all of which yield insoluble trivalent rare earth fluoride compounds when fluorinated in an HF acid solution in accordance with the invention. The mixed valance rare earth oxides such as Tb₄ O₇, Pr₆ O₁₁, Ce₄ O₇, etc. may yield soluble fluoride compounds when fluorinated in an HF acid solution in accordance with the invention and may not be appropriate for use herein without prior processing of the solutions of these materials.

As will become apparent below, the Fe₂ O₃ particulates and the RE₂ O₃ particulates are mixed in proportions selected to produce upon hydrofluorination in accordance with the invention an insoluble rare earth/iron fluoride compound usable as a reactant in the thermite reduction process and other metallothermic processes. To this end, the starting particulate mixture generally will comprise about 5 weight % to about 40 weight % Fe₂ O₃ and the balance RE₂ O₃. A preferred starting particulate mixture comprises about 13 weight % to about 30 weight % Fe₂ O₃ and the balance RE₂ O₃. The Fe₂ O₃ and RE₂ O₃ particulates in the selected proportions are placed in a conventional V-type mixer and intimately mixed/blended to obtain a uniform distribution of the particulates throughout.

The starting Fe₂ O₃ /RE₂ O₃ particulate mixture is then placed in a suitable reaction vessel or container (e.g. in a polyethlylene, Teflon, or other plastic or plastic coated container) and reacted with an excess amount of HF acid solution added to the vessel to fluorinate the oxides. The HF acid solution may be added to the vessel as a single charge or, preferably, as a plurality of successive smaller incremental charges. The vessel contents are stirred to form a slurry of the RE₂ O₃ /Fe₂ O₃ particulates and the HF acid solution.

The HF acid is added in excess of the stoichiometric amount needed to fluorinate the RE₂ O₃ and Fe₂ O₃. In particular, the HF acid is added in an excess amount effective to produce the hydrofluorination reaction in accordance with the equation:

    RE.sub.2 O.sub.3 /Fe.sub.2 O.sub.3 +6HF→REF.sub.3 /FeF.sub.3 xH.sub.2 O                                                (I)

where x is typically 0.5 to 3. More specifically, the HF acid is added in an excess amount that is effective to form the insoluble REF₃ /FeF₃ ·xH₂ O compound (precipitated salt) but that is ineffective (insufficient) to form soluble complex iron fluorides (e.g., iron oxyfluorides) and soluble iron fluorides. To this end, the HF acid is present from about a 5% to about a 25% excess of the stoichiometric amount necessary to hydrofluorination the oxides. In the Examples set forth below, the HF acid is present in about a 10 to 20% excess of the stoichiometric amount.

The invention is not limited to any particular concentration of HF acid solution. For example, from about 20 wt % to about 70 wt % aqueous HF solutions may be used in the invention to carry out the reaction (I) set forth above. Preferably, at least a 30 wt % to 60 wt % HF solution is used. A commercially available 48 wt % HF acid solution has been successfully employed in the method of the invention to carry out the reaction (I).

When the commercially available 48 wt % HF solution is employed to carry out reaction (I), the temperature of the reaction is maintained from about a minimum temperature of 100° C. to a maximum temperature of about 135° C. (boiling point of the 48 wt % HF solution). The reaction temperature can be conveniently controlled within the desired range by cooling the reaction vessel and/or by adding the HF acid solution in controlled/timed increments.

The reaction (I) yields an insoluble, hydrated rare earth/iron fluoride compound (precipitated salt) represented for convenience by the formula REF₃ /FEF₃ ·xH₂ O wherein the FeF₃ is present in solid solution (e.g., ionically as Fe⁺⁺⁺ and F⁻) in the REF₃ host crystal lattice. The "solid solution" nature of the fluoride compound is evidenced by the color of the fluoride compound in both the hydrated and dried forms vis-a-vis the color of the RE₂ O₃ and Fe₂ O₃ particulates and also by the X-ray diffraction pattern of the dried REF₃ /FeF₃ compound vis-a-vis the X-ray diffraction patterns of dried REF₃ and dried FeF₃ as will be described in more detail below.

The hydrated fluoride compound (REF₃ /FeF₃ ·xH₂ O) is insoluble in the HF acid solution such that the precipitate thereof can be readily separated by well established decantation/filtering techniques. After separation, the hydrated fluoride compound is dried to powder cake form. In particular, the hydrated fluoride compound can be dried by heating in a stream of anhydrous hydrogen fluoride gas to a temperature of about 500° C. to 600° C. for a time to remove adhered water and water of hydration. Alternately, the hydrated fluoride compound can be dried by room temperature air drying for about 15 hours to remove adhering moisture and slowly drying to 500° C. to prevent pyrohydrolysis of the fluorides to the oxyfluoride or oxide form. The dried REF₃ /FeF₃ compound typically contains only up to about 0.4 weight % of water, preferably less than about 0.2 weight % water.

An X-ray diffraction pattern of the dried REF₃ /FeF₃ compound corresponds substantially to the X-ray diffraction pattern of dried REF₃ compound with no indication of a pattern corresponding to FeF₃. For example, compare the X-ray diffraction pattern of FIG. 2 for the dried NdF₃ /FeF₃ compound prepared in accordance with Example 1 with the diffractions patterns of dried NdF₃ (FIG. 5) and dried FeF₃ (FIG. 6). To this same end, also compare the X-ray diffraction pattern of the dried DyF₃ /FeF₃ compound (FIG. 3) prepared in accordance with Example 2 versus the X-ray diffraction pattern of dried DyF₃ (FIG. 7) and the X-ray diffraction pattern of the dried GdF₃ /FeF₃ compound (FIG. 4) prepared in accordance with Example 3 versus that of dried GdF₃ (FIG. 8).

These diffraction patterns indicate that the FeF₃ is present in solid solution (e.g., ionically as Fe⁺⁺⁺ and F⁻) in the REF₃ host crystal lattice. Although the specific lattice positions occupied by the FeF₃ in the REF₃ host crystal lattice are not accurately known, the X-ray diffraction patterns of the dried fluoride compounds of the invention are nevertheless effective to indicate that the FeF₃ is present in solid solution in the REF₃ host crystal lattice and does not exist as a separate, distinct compound (which would exhibit its own distinct X-ray diffraction pattern).

The dried REF₃ /FeF₃ fluoride compound of the invention is usable as a reactant in the thermite reduction process in lieu of the separate rare earth fluoride reactant and iron fluoride reactant (thermal booster) employed heretofore in practicing that process. For example, the REF₃ /FeF₃ fluoride compound is mixed with a reducing metal (e.g., Ca) and the resulting reaction mixture is heated to a temperature to form an RE/Fe alloy in accordance with U.S. Pat. No. 4,612,047, the teachings of which are incorporated herein by reference. To this end, the dried REF₃ /FeF₃ fluoride compound comprises about 5 weight % to about 40 weight %, preferably about 13 weight % to about 30 weight, % FeF₃ and the balance REF₃. However, the compound composition employed will depend on the particular rare earth/iron alloy to be prepared by the thermite reduction process. The composition of the rare earth/iron fluoride compound can be controlled by appropriate selection of the initial proportions of Fe₂ O₃ and RE₂ O₃ in the starting particulate mixture.

The REF₃ /FeF₃ compound produced by the wet process described above provides a lower cost reactant for the thermite reduction process than was available heretofore. Moreover, use of this compound reduces the number of reactants which must be made and handled to carry out that process and also reduces the number of processing steps required to prepare the reactants. As a result, the overall cost of practicing the thermite reduction process to manufacture rare earth/iron alloys is reduced.

The following Examples are offered to illustrate the invention in further detail without limiting the scope thereof.

EXAMPLE 1

A starting particulate (powder) mixture was prepared by mixing 99 grams of Nd₂ O₃ (-30 mesh) and 21 grams of Fe₂ O₃ (-30 mesh) such that the mixture comprised about 17.5 weight % Fe₂ O₃ and the balance Nd₂ O₃. This starting powder mixture was placed in a Teflon beaker and 118 ml of 48 weight % aqueous HF acid solution were added and stirred with a plastic rod. The HF acid solution was added in a 10% excess of the stoichiometric amount necessary to hydrofluorinate the oxides without forming soluble Fe complex ions. The temperature of the resulting slurry was maintained at about 100° C. as the reaction (I) (set forth above) of the Nd₂ O₃ /Fe₂ O₃ and the HF solution took place.

After the reaction was complete (e.g., after about 20 minutes), the insoluble, hydrated NdF₃ /FeF₃ compound (precipitated salt) was separated by established decantation techniques and then dried by heating in a stream of anhydrous hydrogen fluoride gas at 500-600° C. for 4 hours. A dried NdF₃ /FeF₃ compound in powder form was thereby produced. The dried NdF₃ /FeF₃ compound comprised 56.46 weight % Nd, 10.69 weight % Fe and 33.15 weight % F. This analysis corresponds to an NdF₃ -20.3 weight % FeF₃ compound.

X-ray diffraction patterns were made of this dried NdF₃ -20.3 weight % FeF₃ compound and also of dried NdF₃ and dried FeF₃ for comparison purposes. The X-ray diffraction patterns were obtained by using a Scintag powder diffractometer and Cu K alpha radiation.

The diffraction pattern of the NdF₃ -20.3 weight % FeF₃ compound corresponded substantially to that of the NdF₃, e.g., compare FIGS. 2 and 5. This correspondence indicated that the FeF₃ is present in solid solution (e.g., as Fe⁺⁺⁺ and F⁻) in the NdF₃ host crystal lattice. In other words, at this particular concentration (i.e., 20.3 weight % FeF₃), the FeF₃ is not present as a separate, distinct compound.

The "solid solution" nature of the NdF₃ -20.3 weight % FeF₃ compound in both the hydrated and dried forms is also confirmed by the pinkish color of the hydrated and dried compound as compared to the blue color of the Nd₂ O₃ particulates and the barn red color of Fe₂ O₃ particulates of the starting particulate mixture.

Approximately 5 kilograms of the dried NdF₃ -20.3 weight % FeF₃ compound were prepared in the manner described above and employed as a reactant in the thermite reduction process (carried out in accordance with U.S. Pat. No. 4,612,047 with the exception that the dried NdF₃ -20.3 weight % FeF₃ compound was used in lieu of the separate NdF₃ reactant and FeF₃ reactant described in the patent). Thermite reduction of the NdF₃ -20.3 weight FeF₃ compound yielded a Nd-15 weight % Fe alloy.

EXAMPLE 2

A starting particulate (powder) mixture was prepared by mixing 100 grams of Dy₂ O₃ (-30 mesh) and 42.8 grams of Fe₂ O₃ (-30 mesh) such that the mixture comprised about 30 wt % Fe₂ O₃ and the balance Dy₂ O₃. The starting powder mixture was placed in a Teflon beaker and 160 ml of 48 weight % aqueous HF acid solution were added and stirred with a plastic rod. The HF acid solution was added in a 20% excess of the stoichiometric amount necessary to hydrofluorinate the oxides without forming soluble Fe complex ions. The temperature of the resulting slurry was about 110° C. as the reaction (I) of the Dy₂ O₃ /Fe₂ O₃ and the HF solution took place. After the reaction was complete, the mixture was left to stand for about 2 hours after which time it was white in color. The insoluble, hydrated DyF₃ /FeF₃ compound (precipitated salt) was separated by established decantation techniques and dried by heating in a stream of anhydrous hydrogen fluoride at 600° C. for about 6 hours. A dried DyF₃ -34 wt % FeF₃ compound in powder form was thereby produced.

X-ray diffraction patterns were made of the dried DyF₃ -34 wt % FeF₃ compound and also dried DyF₃ and dried FeF₃ for comparison purposes. The diffraction pattern of the DyF₃ -34 wt % FeF₃ compound corresponded substantially to that of DyF₃, e.g., compare FIGS. 3 and 7. This correspondence indicated that the FeF₃ is present in solid solution in the DyF₃ host crystal lattice. In other words, at this particular concentration (i.e., 34 wt % FeF₃), the FeF₃ is not present as a separate, distinct compound.

EXAMPLE 3

A starting particulate (powder) mixture was prepared by mixing 100 grams of Gd₂ O₃ (-30 mesh) and 20.9 grams of Fe₂ O₃ (-30 mesh) such that the mixture comprised about 17 wt % Fe₂ O₃ and the balance Gd₂ O₃. The starting powder moisture was placed in a Teflon beaker and 122 ml of 48 wt % aqueous HF acid solution were added and stirred with a plastic rod. The HF solution was added in a 20% excess of the stoichiometric amount necessary to hydrofluorinate the oxides without forming soluble Fe complex ions. The temperature of the resulting slurry was about 110° C. as the reaction (I) of the Gd₂ O₃ /Fe₂ O₃ and the HF took place. After the reaction was complete,, the mixture was left to stand for about 2 hours after which time it was white in color. The insoluble, hydrated GdF₃ /FeF₃ compound (precipitated salt) was separated by established decantation techniques and dried by heating in a stream of anhydrous hydrogen fluoride for about 6 hours at 600° C. A dried GdF₃ -20.0 wt % FeF₃ compound in powder form was thereby produced.

X-ray diffraction patterns were made of the dried GdF₃ -20.0 wt % FeF₃ compound and also dried GdF₃ and dried FeF₃ for comparison purposes. The diffraction pattern of the GdF₃ -20.0 wt % FeF₃ compound corresponded substantially to that of GdF₃, e.g., compare FIGS. 4 and 8. This correspondence indicated that the FeF₃ is present in solid solution in the GdF₃ host crystal lattice. In other words, at this particular concentration (i.e., 20.0 wt % FeF₃), the FeF₃ is not present as a separate, distinct compound.

While the invention has been described in terms of specific embodiments thereof, it is not intended to be limited thereto but rather only to the extent set forth in the following claims. 

What is claimed is:
 1. A method of making a fluoride compound, comprising the steps of:a) preparing a particulate mixture of Fe₂ O₃ and RE₂ O₃ where RE is a rare earth element, and b) reacting the particulate mixture with HF acid in an amount exceeding the stoichiometric amount needed to hydrofluorinate the Fe₂ O₃ and RE₂ O₃, the excess amount of said HF acid being effective to yield an insoluble fluoride compound comprising REF₃ and FeF₃ present in solid solution in the REF₃ crystal lattice.
 2. The method of claim 1 including the further step of drying said fluoride compound produced in step (b), the compound in dried form exhibiting an X-ray diffraction pattern corresponding substantially to that exhibited by REF₃.
 3. The method of claim 1 wherein the particulate mixture comprises about 5 weight % to about 40 weight % Fe₂ O₃ and the balance RE₂ O₃.
 4. The method of claim 3 wherein the particulate mixture comprises about 13 weight % to about 30 weight % Fe₂ O₃ and the balance RE₂ O₃.
 5. The method of claim 1 wherein the HF acid is present in about 5% to about 25% excess of said stoichiometric amount.
 6. The method of claim 5 wherein the HF acid is present in about a 10 to 20% excess of said stoichiometric amount.
 7. The method of claim 1 wherein the HF acid comprises a 48 weight % HF aqueous solution.
 8. The method of claim 7 wherein the particulate mixture is reacted with the HF solution at a reaction temperature up to about 135° C.
 9. The method of claim 8 wherein the reaction temperature is maintained from about 100° C. to about 135° C.
 10. The method of claim 1 wherein the particulate mixture is reacted with the HF acid by adding incremental amounts of the HF acid to a slurry of said mixture and said acid.
 11. The method of claim 1 wherein the rare earth element is selected from the group consisting essentially of Nd, Dy, Gd, Ho, Er, Lu, La, Sc and Y.
 12. In a thermite reduction process for making a rare earth/iron alloy, the steps of (a) forming a reaction mixture of a reducing metal and a fluoride compound comprising REF₃, where RE is a rare earth element, and FeF₃ present in solid solution in the REF₃ crystal lattice, said compound comprising about 5 weight % to about 40 weight % FeF₃ and the balance REF₃ and (b) heating the reaction mixture to a temperature to form an RE/Fe alloy.
 13. The process of claim 12 wherein the rare earth element is selected from the group consisting essentially of Nd, Dy, Gd, Ho, Er, Lu, La, Sc and Y.
 14. The process of claim 12 wherein the fluoride compound is made by1) preparing a particulate mixture of Fe₂ O₃ and RE₂ O₃ where RE is a rare earth element, 2) reacting the particulate mixture with HF acid in an amount exceeding the stoichiometric amount needed to hydrofluorinate the Fe₂ O₃ and RE₂ O₃, the excess amount of said HF acid being effective to yield an insoluble fluoride compound comprising REF₃ and FeF₃ present in solid solution in the REF₃ crystal lattice, and 3) drying the fluoride compound produced in step (2).
 15. A fluoride compound comprising REF₃, where RE is a rare earth element, and FeF₃ present in solid solution in the REF₃ crystal lattice.
 16. A fluoride compound comprising REF₃, where RE is a rare earth element, and FeF₃ present in solid solution in the REF₃ crystal lattice as indicated by an X-ray diffraction of said compound corresponding substantially to that exhibited by REF₃.
 17. The compound of claim 15 comprising about 5 weight % to about 40 weight % FeF₃ and the balance REF₃.
 18. The compound of claim 17 comprising 13 weight % to about 30 weight % FeF₃ and the balance REF₃.
 19. The compound of claim 16 wherein the rare earth element is selected from the group consisting essentially of Nd, Dy, Gd, Ho, Er, Lu, La, Sc and Y.
 20. A fluoride compound useful as a reactant in the thermite reduction process, comprising NdF₃ and FeF₃ present in solid solution in the NdF₃ crystal lattice as indicated by an X-ray diffraction pattern of said compound corresponding substantially to that exhibited by NdF₃, said compound comprising about 5 weight % to about 40 weight % FeF₃ and the balance NdF₃.
 21. A fluoride compound useful as a reactant in the thermite reduction process, comprising DyF₃ and FeF₃ present in solid solution in the DyF₃ crystal lattice as indicated by an X-ray diffraction pattern of said compound corresponding substantially to that exhibited by DyF₃, said compound comprising about 5 weight % to about 40 weight % FeF₃ and the balance DyF₃.
 22. A fluoride compound useful as a reactant in the thermite reduction process, comprising GdF₃ and FeF₃ present in solid solution in the GdF₃ crystal lattice as indicated by an X-ray diffraction pattern of said compound corresponding substantially to that exhibited by GdF₃, said compound comprising about 5 weight % to about 40 weight % FeF₃ and the balance GdF₃. 